Tipping is optional. You are free to add a tip, and drivers are free to accept tips. How do I leave my driver a tip? The easiest way to tip your driver is through the app.
Do Uber drivers know when you tip?
Uber drivers can't tell if you tipped until after they rate you or when you leave the tip, which ever comes last. There is no secret app or trick.
What happens if you don't tip your Uber driver?
No, drivers are forced to rate you before they can do anything else after giving you a ride. They can't check if you tipped first or accept another ride first or anything else. They're also required to explain what was wrong if it is less than a 5 star rating.